The present invention relates to a multiplexed system which can be switched, as a whole, from a standby system to an active system and vice versa in such a system as transmission equipment wherein its each portion is multiplexed and a high degree of reliability is required.
If a fault occurs in a transmission path of an active system in a SDH (Synchronous Digital Hierarchy) transmission equipment having a duplexed transmission path, the equipment detects the fault automatically, thereby allowing a system to switch from an active transmission path to a standby transmission path. Control procedures for such switching are stipulated in recommendations of ITU-T (G.707,783) which has standardized operating procedures for the transmission equipment.
When a fault occurs, the transmission path or each of the transmission equipment and peripheral devices of the transmission equipment are adapted to send and receive signals for control of the switching among them, by using K1/K2 bytes of SOH (Section overhead) contained in bytes of APS (Auto protection switch) defined in the above stipulation, in order to execute switching operations of a system in cooperation with each other.
As triggers used for executing switching of a system, alarms for a portion and/or equipment, alarm from associated equipment, and an instruction for switching from high order equipment are included for use.
Two pieces of transmission equipment are placed opposite to each other with a group of transmission paths between. A high order equipment is used to monitor and control operations of two pieces of the transmission equipment. Out of the two pieces of the equipment, one is called xe2x80x9cself equipmentxe2x80x9d and the other is called xe2x80x9cother equipmentxe2x80x9d. Here, the self equipment is mainly described herein.
The section represents a group of transmission paths used to connect the self equipment to the other equipment. The alarm for the section is an alarm outputted for the section to require the switching between transmission paths of a standby system and of an active system. The alarm for the equipment is an alarm outputted for the self equipment to require the switching between appropriate blocks of a standby system and of an active system within the equipment.
The alarm for the associated equipment is an alarm by which the associated equipment requests the self equipment to switch the system in harmony with the associated equipment when the associated equipment is switched from its standby system to its active system. An instruction for switching from high order equipment is an alarm by which the high order equipment requests the self equipment to be switched between a standby system and an active system when the high order equipment detects some faults.
In either case, the alarm is written into memory for writing of the alarm embedded in the self equipment. The firmware used to control the self equipment polls this memory. This enables reading of alarms from memory at a predetermined interval.
When the firmware reads the alarm out, it recognizes the source of an alarm by an address used to read the alarm from memory or by information about attributes of the alarm and renews a state transition diagram. The state transition diagram is stored in memory within the self equipment and holds the information about the state of each of the equipment parts, e.g., as to whether each part of the equipment is in the standby state or in the active state. If the firmware rewrites the information about the state, each of the equipment switches the state in accordance with the rewritten information. If a fault should be detected in a device which has been in the active state, the information about the state is adapted to be switched to the standby system accordingly. Then, a device having the same function as a device in an active state which has been in the standby state is switched to the active system.
However, there have been problems to be resolved in the related art described above.
That is, the firmware used to control the state transition of each portion of the transmission equipment reads, by polling, each alarm written in the said memory at a predetermined time interval T. However, because it is after a time T that the part polled immediately before the alarm is written is polled, there is a case where a maximum T time""s delay in the timing of the alarm collection occurs. This may lead to a problem that the time required from when the fault is recognized by the firmware following the output of the alarm to when the switching of corresponding parts between a standby system and an active system is completed is prolonged.
Also, if the state of each of the self equipment part should be totally managed by the state transition table, renewing and management of the table are made complicated, imposing a huge load on a processor.
The present invention has been made to resolve the above problems. The first object of the present invention is to provide a multiplexed system having a group of blocks of an active system and of a standby system wherein parts used to control the state transition are made hardware-controlled as much as possible, thereby taking a load off the firmware.
The second object of the present invention is to provide a multiplexed system wherein the state transition is speeded up by making hardware-controlled, as much as possible, parts used to control the state transition with each block being linked with each other.